Snubberized LM338 regulated PSU problem

[Puffin]

I don't know if this is relevant, but I did not have the required resistor values for a couple of the positions and put two 1/4w metal films in series. Would this give them more power handling? (I think not?)

I would need to open up one of my amps to see which ones these are and to measure the reg modules. If you would like me to do this I will.

Rob.

[Nuuk]

Thanks Rob but I don't think the resistors are the problem. Much more likely it's the start up as abraxalito has suggested. What traffo are you using (I seem to remember it was a 250VA)? Mine is a 400VA and I am wondering if that is the vital difference between our two circuits. I know that some recommend a soft-start for anything over 300VA.

[Puffin]

My tranny is no more than 180VA (from memory)

[Nuuk]

So I wonder if that IS the crucial factor?

Another thought I had is that when I tested the regulator circuit, and then the amp for the first time, I used a light-bulb tester. It was when I removed the tester to play music that things went wrong.

[abraxalito]

Originally Posted by Nuuk

Do you mean as in the top of bottom diagram?

Yes, the top one is what I meant.

The bottom one is useful if you can't afford the voltage drop of the series R but it still lets HF noise pass through the regulator unchecked. If you were to use this case the R should be a normal carbon or metal film, not wirewound. It doesn't need to dissipate much power.

Using the lightbulb in series with the input slows down the voltage rise considerably - it would most likely prevent the condition where over 40V appears across the reg.

[Nuuk]

Thanks again. It looks as if I would need to build in a soft-start to use this traffo with the LM338's.

[abraxalito]

You could try something a bit simpler - like a zener diode across the regulator. Say a 1.3W type, 33V. I have no experience of such solutions, but it sounds like it might be worth a try to limit the transient overvoltage.

[Nuuk]

When you say "across the regulator" do you mean across the input and output pins?

[abraxalito]

Yes, that's the idea. With a zener between reg 'in' and reg 'out' the output can't easily get more than 33V below the input. The zener voltage could even be much lower, like 12V. Just you'll blow the zener very quickly if you short the output, and the reg may follow very shortly after...

[Puffin]

Just remembered

I use a soft start on my monoblocks (relay triggered)

EDIT: Not speaker protection as originally posted. LS are not wired to it.